The use of vacuum forming is a recognized process, particularly in the automotive industry. For example, automobile surfaces such as consoles, dashboards, door panels and other interior surfaces can be covered with a thermoplastic material. The substrates can be any commonly known materials, such as acrylonitrile-butadiene-styrene terpolymer (ABS) or fiberboard. The process includes coating the substrate with a laminating adhesive, heating a thermoplastic material, such as a polyvinyl chloride flexible trim vinyl, past its softening point and draping the flexible vinyl onto the substrate. A vacuum is then applied through the substrate to pull the flexible vinyl in a contoured fashion onto the substrate.
Use of organic solvent-based adhesives in vacuum forming is common in the industry. However, such adhesives present work hazards typically associated with organic solvents. A water-based thermoforming adhesive is disclosed in U.S. Pat. Nos. 4,762,880 and 4,853,061. These patents disclose the use of an adhesive having an aqueous aromatic polyurethane emulsion or dispersion, a water dispersible crosslinking agent, such as an unblocked organic isocyanate compound. The compositions disclosed in these patents are only stable for several hours and within about seven hours, recognizable performance decreases are observed. Stability of adhesive compositions is an important factor in determining their usefulness. For example, compositions having stability comparable to those disclosed in the above-identified patents must be stored in such a manner that the reactive components are separate and only mixed immediately prior to application. Such handling requirements impose additional costs in use. Moreover, such compositions are particularly uneconomical, if for example, a stoppage is encountered on a production line. In such an instance, an entire batch of adhesive can react sufficiently during the delay to require disposal of the batch.
In view of the foregoing discussion, there is a need for a single package water-based vacuum forming laminating adhesive composition which is highly stable and provides excellent adhesive properties.